v0.15.0

What's Changed

• Improve errors for course_library module by @martinmr in #227
• Return proper errors in blacklist module by @martinmr in #228
• Do not return error if there's a missing index reader by @martinmr in #229
• Fix code coverage by @martinmr in #230
• Return proper errors in repository_manager module by @martinmr in #231
• Return proper errors in course_library module by @martinmr in #232
• Improve errors in practice_stats module by @martinmr in #233
• Improve errors for review_list module by @martinmr in #234
• Simplify errors for blacklist and practice_stats modules by @martinmr in #235
• Improve errors for scheduler module by @martinmr in #236
• Further simplify error handling by @martinmr in #237
• Simplify errors for repository_manager module by @martinmr in #238
• Optimize exercise scheduling by @martinmr in #239
• Improve errors for graph module by @martinmr in #240

Full Changelog: v0.14.4...v0.15.0